Linux was missing a default value for DEV_DSKDIR. Set it to /dev.
Fix resulting fallout.
SLICE_PREFIX seems like a good candidate for including in the defaults.
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes#9898